This invention relates generally to power generation and more specifically to methods and apparatus for detecting islanding of a power generation facility connected to a grid. The methods and apparatus are particularly applicable to wind farms, but are not necessarily limited thereto.
An open circuit, such as caused by opening of a switch, circuit breaker, or fuse, in a radial connection between a windfarm and a grid power network, can leave the windfarm isolated from the grid. This isolation is called “islanded operation,” and is forbidden in several grid codes. Similarly, the opening of a circuit breaker or switch at a windfarm substation, resulting in isolation of a collector circuit from the substation, islands the wind turbine on that collector circuit from the substation and from the grid. Although the windfarm is isolated from grid, it might still be connected to consumers outside of the wind farm. Islanding can result in severe stress on equipment, including high voltages and can be dangerous to maintenance personnel. Avoidance of severe equipment stress can require very fast detection of the islanded condition.
U.S. Pat. No. 6,810,339 B2 describes some known methods for eliminating the islanding problem. These include passive methods such as under or overvoltage, and under and over frequency shutdown trips of converter equipment if voltage or frequency exceed certain predefined limits. Additional schemes use “unstable frequency” or “active frequency drift, or changing the real or reactive power output of a converter. A method said to be superior to these is disclosed in which a converter detects a change of frequency, up or down, and causes an accelerated frequency shift in the same direction to quickly trip an under/over frequency limit.
U.S. Pat. No. 6,815,932 B2 describes a method for controlling a generator system connected to an electric power system so as to avoid the unintentional islanding of the generator. The method uses a combination of frequency characteristic thresholds and an active phase angle destabilization technique to destabilize well or perfectly matched islands.
Nevertheless, reliable detection of islanding by passive observation of voltage, current, frequency, and/or phase angle change has proven difficult. These detection methods are often incapable of very fast detection, are susceptible to false operation caused by grid events, and can be confounded by balanced or near-balance between power generated by the windfarm and the power demand within the area of the grid that is islanded with the windfarm. Detection by monitoring switch status is complicated, requiring an expensive communication system. Often the status of multiple switching devices is needed, along with logic to account for multiple paths of interconnection.